This invention relates to a sawtooth oscillator comprising a first capacitor, a current source having an output coupled to the first capacitor for supplying a charge current to the first capacitor, and means for discharging the first capacitor during a discharge period in response to a voltage on the first capacitor. Such an oscillator is generally known from manuals and is used in commercially available Timer ICs of the 555 type described in the Signetics Integrated Circuits Data Handbook, 1978. The first capacitor is charged by the current source. Once the voltage on the first capacitor exceeds a first threshold, the first capacitor is discharged by the discharging means, usually a transistor connected in parallel with the first capacitor, until a second threshold voltage is reached at which the discharge is terminated. The first capacitor is then charged again by the current source. The voltage on the first capacitor then has a sawtooth shape. The discharge period is the period of time elapsing between the capacitor reaching the first threshold value and reaching the second threshold value. The charge period is the period of time elapsing between the capacitor reaching the second threshold value and reaching the first threshold value. The overall period of time of a charge and discharge cycle is the sum of the discharge and charge periods. This sum determines the frequency of the oscillation. Due to tolerances in the discharging means the discharge period is not fixed. If the discharge transistor is relatively small, it will switch relatively rapidly, but has a relatively large internal resistance. In the case of a relatively large discharge transistor the reverse is true. As a result of these tolerances the accuracy of the oscillation frequency is adversely affected.